The invention relates to high voltage switching apparatus and, more particularly, to high voltage switching apparatus using a field effect transistor pulse control circuit for a synthetic aperture radar transmitter.
It is known in the art to provide high voltage pulse apparatus using field effect transistors (FETs). For example, U.S. Pat. No. 4,425,518 discloses high speed FET transistor pulse apparatus with a quick rise time. Such apparatus provides excellent performance in applications such as pulse doppler radar transmitters.
The apparatus disclosed in U.S. Pat. No. 4,425,518 exhibits disadvantages when used in certain other applications. For example, a certain amount of ringing during the pulse duration between the rising and falling edges of the pulse is acceptable in pulse doppler applications but can greatly limit performance in synthetic aperture radar applications. It is therefore an object of the invention to provide apparatus exhibiting extremely flat output pulses with a minimum of ringing.
In addition, for certain applications involving extremely fast rise time pulses, unsatisfactory results were sometimes obtained. It has subsequently been discovered that some of these effects are due to the phenomenon of Miller integration in which the internal capacitance between the drain and gate of field effect transistors (FETs) forms a negative feedback circuit which limits the rise and fall times of signals applied to such devices. It is therefore an object of the invention to provide a high voltage pulse apparatus which does not exhibit performance limitations caused by Miller integration.
Certain types of high-power transmitting tubes, such as travelling wave tubes, exhibit considerable variation in their grid requirements for a B+ power supply. In prior art arrangements, such variation was accommodated by providing an adjustable B+ supply or by substituting entirely different B+ supplies if it was determined through testing that such supplies were required. It would therefore be desirable to provide pulse apparatus for use with tubes exhibiting such variation in B+ supply requirements which did not require separate or adjustable B+ supplies.
In radar applications employing high voltage pulse apparatus, a first portion of the circuit is conventionally constructed using a low voltage reference potential and a second portion of the circuitry constructed using a high voltage reference potential. The first circuit portion is commonly referred to as the "ground deck" and the second portion is commonly referred to as the "floating deck." It is necessary to transmit control information from the ground deck to the floating deck. One method employed in the prior art was to transmit a control pulse consisting of a replica of the desired high voltage output pulse from the ground deck to the floating deck using video transformers. Such apparatus had the disadvantage of coupling undesired noise between the ground deck and the floating deck. Other prior art apparatus transmitted separate ON and OFF pulses from the ground deck to the floating deck and then constructed the output pulse entirely on the floating deck. The control pulses were sometimes transmitted between the ground deck and the floating deck using toroid transformers with wound or potted dielectric material. The former design provided difficulties in fabrication, whereas the latter often exhibited problems when subjected to extreme environmental conditions. Another method of transmitting the control pulses between the ground deck and floating deck in the prior art employed optoisolator circuits. While such circuits prove useful in certain applications, the variable delays exhibited between multiple optical isolators cause difficulties in other applications. It is therefore an object of the invention to provide high voltage switching apparatus in which control information is transmitted between the ground deck and floating deck while avoiding the above-discussed difficulties.